Sporadic violence across Syria casts doubt over UN plans to expand a team of observers to the country, US envoy to the UN Susan Rice said.

The US Ambassador said the continued violence, in violation of a ceasefire, was "gravely concerning".

She said:

"Should the violence persist and the ceasefire, or cessation of violence more aptly, not hold, that ... will call into question the wisdom and the viability of sending in the full monitoring presence."

Videos purporting to show shelling in Homs earlier today have been posted on social media websites on the same day as UN observers arrive in Syria to monitor the ceasefire. The videos claim to show the neighbourhoods of Juret al Shayah and al-Karabees.

ITV News cannot independently verify the veracity of the footage or the date it was shot.

Activists say Syrian troops are shelling neighbourhoods in the opposition stronghold of Homs, hours after the arrival of six UN observers in Damascus.

The Local Co-ordination Committees and the British-based Syrian Observatory for Human Rights also said two people were killed in Hama in central Syria today when security forces opened fire on their car.

The Syrian Government has said that it cannot be responsible for the safety of UN monitors unless it is involved in all steps on the ground and confirmed the number of monitors to rise to 250 as agreed.

It sas also stressed that it has the right to respond to any acts of aggression by armed groups.